As important basic petrochemical feedstock, lower carbon olefins such as ethylene and/or propylene have attracted a lot of attention from research and development teams to their preparation as well as subsequent separation and purification. In the past lower carbon olefins such as ethylene and/or propylene were primarily prepared by pyrolysis of petroleum hydrocarbon fractions such as naphtha and light diesel, however, in recent years a process for preparing olefins by pyrolysis of oxygenates had been developed due to the gradual short supply of crude oil.
No matter the pyrolysis is of petroleum hydrocarbons or of oxygenates, the resultant pyrolysis gas is always a mixture of complicated ingredients and depending on the process conditions generally comprises lower carbon olefins such as C2-C4 olefins at relative large amounts, also some non-olefin byproducts such as hydrogen, C1-C6 alkanes and little alkyne as well as in the case of pyrolysis of oxygenates some unreacted oxygenates such as alcohol and/or ether etc. Thus, a complicated separation and purification process is necessary to separate and purify such a complicated pyrolysis gas to obtain lower carbon olefins such as ethylene and/or propylene of polymerization grade.
The pyrolysis gas from preparation of lower carbon olefins is generally subjected to a cryogenic separation process, which typically covers three separation schemes, i.e. sequential scheme removing methane firstly, front end deethanizer scheme removing C2 and the lower fractions firstly, and front end depropanizer scheme removing C3 and the lower fractions firstly. In these separation schemes, the pyrolysis gas is generally pretreated, e.g. cooled, compressed, removed of impurities and dried as well as optionally finished, and then further treated to obtain lower carbon olefins of polymerization grade finally. In these separation schemes, when separating methane and hydrogen from C2+ fractions, a cryogenic separation process with high investment cost and energy consumption is necessary. In order to overcome the disadvantages of the cryogenic separation process, newly proposed is a process for separating by absorption the pyrolysis gas from preparation of lower carbon olefins, i.e. separating methane and hydrogen by absorbing C2+ fractions with an absorbent at moderate temperature and pressure.
In the absorption process, mixed hydrocarbons or pure hydrocarbon are generally used as the absorbents to separate methane and hydrogen from C2+ fractions at reasonable operating conditions and minimize the loss of targeted products such as ethylene and/or propylene as possible as can. In order to minimize the concentration of targeted products such as ethylene and/or propylene at the overhead of the absorption column, some measures such as circulating a lot of absorbent or decreasing the temperature of the absorbent are used to increase the absorption capacity, however, all these measures are with high energy consumptions. Thus, a compromise is necessary between minimizing the loss of targeted products such as ethylene and/or propylene and the energy consumption during the process.
Thus, in the art it is still needed to further improve the yield of targeted products such as ethylene and/or propylene and decrease the energy consumption during the separation and purification of the pyrolysis gas from preparation of lower carbon olefins.